Sulfates and Hygiene Products: A Potentially Irritating Combination

On the back of every shampoo and every soap, there is a list of all the chemicals used for their production. Have you ever wondered what these substances are and why there are so many of them in something as simple as soap?

Form versus substance

The quality and the shelf life of soaps and shampoos has been drastically refined in the last several decades. They are stable for a long time, smell great, come in a variety of forms, and for a variety of purposes. It is no wonder that the list of chemicals inside each of them got bigger and more complex. Colours, smells, emulsifiers, surfactants, stabilizers and detergents are but a few notable groups of such chemicals. However, the potential for a part of such a product to be harmful, or just irritating to some, also got exponentially bigger.

SLS and SLES: Two Persistent Compounds

Sodium Lauryl Sulphate (SLS) and its derivate Sodium Laureth Sulphate (SLES) are common surfactants and anionic detergents, meaning they decrease the surface tension and by doing so help remove dirt and oily substances. They are found in many personal care products, ranging from toothpaste, soaps and shampoos, to industrial use cleaning agents. They are inexpensive, and very effective foaming agents, creating rich and long-lasting bubble foam, called lather. Both compounds are derived from non-renewable resources and are not biodegradable. However, SLES is considered to be less harmful to the environment than SLS because SLES can degrade in the presence of sunlight and oxygen, while SLS can persist in the environment for long periods of time.

Figure 1: Chemical structure of sudium lauryl sulphate. The n represents a number of repeating elements inside the bracket. Sodium laureth sulphate hosts the same structure without the repeating elements.

Safe, but not without risk

Although both SLS and SLES are considered safe for human use, they are not without risk. As stated, they are labelled „generally recognized as safe“ (GRAS) ingredients according to the USFDA. Moreover, both are considered not carcinogenic in low concentrations. However, as their surfactant feature removes oils from skin and hair, it makes them a potent irritant in some people as well. SLES has been shown to be the milder of the two, but both have been shown to irritate the skin with prolonged use and worsen skin problems such as skin hypersensitivity, dry patches, seborrhoeic dermatitis, and other kind of dermatitis. They have also been shown to promote hair folicle damage and weakening the hair, as they can remain around the hair stem for longer and disrupt the surrounding skin. Additionally, if ingested orally, both can lead to formation or worsening of mouth ulcers.

Another concern is the purity of SLS and SLES, due to their tendencies to be polluted with ethylene oxide and 1,4-dioxane, both of which are known carcinogens. Moreover, ethylene oxide can also harm the nervous system and has been classified as a possible developmental toxicant based on evidence that it may interfere with human development.

1,4-dioxane can be removed from cosmetics during the manufacturing process by a process called vacuum stripping, but there is no regulatory mandate for providing such a proof to the consumer.

Caution: Sensitive Skin

Both SLS and SLES are a great tool for cleaning all sort of surfaces, from skin and hair, to clothes and surfaces. They are such a great product precisely because they are aggressive as a surfactant, cheap and easy to produce in massive quantities. Unfortunately for the likes of us with sensitive skin, they can be too aggressive and aggravate our skin even more. Luckily, there are other options to choose from, such as sulphate-free soaps and shampoos that employ some other form of anionic detergent.

Past and future research

There is no research going on with SLS nor SLES, and the last safety assessment study, published in the International Journal of Toxicology, was done in 1983, when the use of SLS and SLES was miniscule compared to today's standards. It would be a good opportunity to revisit such claims today, more than 40 years later, and examine SLS and SLES in the era of higher concentrations and better research equipment.